1. Description of the Related Art
Recently, with high functionality and compactness of an apparatus, such as an imaging optical system or a projection optical system, there has been a growing demand for an optical glass having a high refractive index as a material of an effective optical element.
The optical glass (hereinafter simply referred to as a “glass”) having the high refractive index usually contains a large amount of high refractive index components, for example, Ti, Nb, W, Bi or the like, as glass components. Since these components are easily reduced during the melting of the glass and the reduced components absorb short-wavelength light in a visible light region, they may cause an increase in coloring (hereinafter sometimes referred to as “reduction color”) of the glass.
In order to solve the problem, technique of thermally treating the colored glass again and thereby decreasing the reduction color has been known. However, this method is problematic in that, if the coloring of the glass is too dark before the thermal treatment, a sufficient decrease in the reduction color may not be achieved even by the thermal treatment. Hence, a glass composition having a larger amount of high refractive index components makes it difficult to decrease the reduction color.
In contrast, Patent Document 1 has proposed technique that is intended to decrease the reduction color of a glass in a melting process, by producing an oxidizing gas from a glass raw material in the melting process and thereby inhibiting a high refractive index component from being reduced. According to this technique, it is possible to manufacture an optical glass that is decreased in reduction color, even if the glass has a composition containing a large amount of high refractive index components, like a borate bismuth-based glass.
However, in the case of manufacturing a phosphate-based optical glass using this technique, a problem where a noble metal material constituting a melting vessel dissolves in the molten glass may become serious.
Generally, a noble metal material, such as platinum having high heat resistance or corrosion resistance, is widely used as a material constituting the melting vessel such as a crucible. A melting instrument made of the noble metal material is needed to mass produce glass that is very high in homogeneity, for example, an optical glass. However, high temperature generated during the melting may sometimes cause noble metal such as platinum to react with oxygen contained in a melting atmosphere (e.g. air) and thereby produce a noble metal oxide (e.g. PtO2 etc.). Such a noble metal oxide dissolves in the glass through a surface of a melting product. Further, when the glass containing a large amount of high refractive index components is melted, the noble metal material may be oxidized and dissolve in the molten glass in the form of noble metal ions (e.g. Pt4+ etc.).
The noble metal ions (containing the noble metal oxide) dissolved in the molten glass absorb visible light, thus leading to an increase in coloring (coloring derived from the noble metal ions) in the optical glass as a final product. In addition, the optical glass containing noble metal such as platinum causes deterioration in transmittance of the glass due to ultraviolet radiation (solarization). Therefore, when the optical glass is manufactured, it is required to inhibit the noble metal from being eluted into the molten glass.
Typically, the higher the melting temperature of the glass is, the more the amount of the noble metal dissolved in the molten glass is. Thus, when the phosphate-based optical glass (having high glass transition temperature Tg) whose melting temperature is required to be set to be higher than that of the borate bismuth-based glass is manufactured, the dissolving of the noble metal in the molten glass becomes a more serious problem.
In the case of manufacturing this phosphate-based optical glass, technique (producing the oxidizing gas from the glass raw material in the melting process) of Patent Document 1 may decrease the reduction color of the glass, but causes the noble metal to be dissolved in the molten glass. Consequently, sufficient improvement in transmittance cannot be expected.
In contrast, Patent Document 2 has proposed technique of achieving both a decrease in a reduction color and a decrease in amount of dissolved noble metal when the phosphate-based optical glass is manufactured. This method may promote the discharge of excess oxygen components in a melting atmosphere by bubbling non-oxidizing gas in the molten glass, and may prevent the noble metal constituting the melting vessel from being eluted in the glass. Further, since this method may appropriately adjust the concentration of an oxygen component in the molten glass, it is possible to prevent the oxygen component from being excessively discharged from the molten glass, and thereby it is also possible to inhibit the reduction of the high refractive index component.
However, in the case of using reduction gas (carbon monoxide etc.) as the non-oxidizing gas, metal ions of Ti, Nb, W, Bi or the like in the molten glass are excessively reduced, and the reduced metal and the noble metal constituting the melting vessel are alloyed, thus undesirably causing the strength and durability of the melting vessel to be considerably lowered. Further, since an inert gas such as Ar is more expensive than other gases, the use of the inert gas as the non-oxidizing gas may lead to an increase in cost.
Furthermore, such a method may decrease the amount of the noble metal dissolved in the molten glass. However, since a melting operation is performed under the melting atmosphere where oxygen partial pressure is low, the reduction of the high refractive index component is performed, so that the coloring of the glass becomes dark. It is impossible to sufficiently mitigate the coloring, even if this glass is subjected to the thermal treatment.
Therefore, a glass (moreover, optical glass of a high refractive index having excellent transmittance) and a method for manufacturing the glass are required, which are capable of drastically decreasing a reduction color of the glass while keeping a noble metal content low.